This invention generally relates to accurately registering a contact mask to a substrate for a subsequent thin-film deposition. More particularly, it relates to an evaporation fixture which accurately registers a plurality of small substrates to a single metal contact mask for use in a thin film deposition process.
The use of a contact mask to control the pattern of material deposition on a semiconductor wafer, or other substrate, is well-known. Openings are provided in the contact mask in a pattern which corresponds to the desired pattern of the material to be deposited on the substrate. The contact mask is then placed against the surface of the substrate and one or more materials are deposited on the substrate through the patterned arrangement of the openings in the mask. Contact masks are frequently used where metallic material is deposited on integrated circuit semiconductor wafers which are thereafter mounted on various electronic devices.
However, prior art techniques for registering contact masks to substrates have been limited to mating a single substrate to a single mask. If more than one substrate is to be patterned on a given deposition run, the prior art teaches that a separate contact mask must be used for each substrate. This becomes incredibly time consuming when thousands of very small individual substrates must be masked. The prior art does not address a situation where multiple small substrates must be aligned and registered to a single contact mask.
Several problems are encountered when multiple small substrates are registered to a single mask, which are either not found with single mask, single substrate arrangements, or are greatly exacerbated with multiple substrates. One of the more critical concerns for a contact mask is that the substrate must be in uniform contact with that mask; otherwise, haloing of the deposited material under the mask will occur on the substrate; i.e., material will be deposited in areas outside those intended. To prevent haloing, the mask should be very flat against the substrate. Assuring good mask contact has been accomplished in the prior art by flexing the contact mask tightly against the flat surface of the substrate. However, this method is not useful when a collection of substrates of a variety of thicknesses and which do not present a uniform and planar face area to flex the contact mask against.
A second problem occurs in aligning each individual chip or substrate to the contact mask in the X-Y plane. This is simply not addressed in the prior art as there is but a single substrate to align to the contact mask. One known strategy for aligning a contact mask to a substrate is the use of several alignment pins and several complementary alignment pin receiving means. Generally, the substrate is seated in the holder and has alignment pins which mate with the alignment receiving means present either on the contact mask or on a mask holder in which the mask is encased. This works well where a single substrate and single mask are used. However, where a plurality of substrates of varying dimensions in length and width, a limited number of alignment pins will not do an adequate job. Other means used in the prior art include multiple cams or clamping pins.
Another problem encountered in contact mask deposition is to separate the individual chip from the contact mask after deposition. A typical film deposited through a contact mask onto a substrate would be a thick metal film with substantial tensile and bonding strength. Where a single substrate is used, it is relatively easy to hold the substrate while the contact mask is peeled off. However, the problems of holding each individual chip in place while a contact mask is removed are much greater.
After the material is deposited in the desired pattern on the substrate, the mask and at least some of the parts of the evaporation fixture must be cleaned. It would be desirable if an evaporation fixture were designed so that minimum number of parts needed to be cleaned to minimize labor and the number of extra parts needed for the evaporation fixture in a manufacturing mode.